1. Field of the Invention
The present invention relates to an ultrasonic diagnostic apparatus which irradiates blood flow with ultrasonic waves and detects frequency shifts (Doppler shifts) of reflected ultrasonic waves from the blood flow to detect the flow direction and velocity.
A type of such ultrasonic diagnostic apparatus is a Doppler flow mapping apparatus for displaying a blood flow image contained within a monochrome tomography image (B mode image) in colors according to its direction and velocity, or more accurately, a B-mode blood flow imaging (referred to as BDF) apparatus. In general, the blood flow in the direction of a piezoelectric transducer is displayed in a red color, the blood flow in the direction opposite to the transducer is displayed in a blue color and the turbulent flow is displayed in a green color. The blood flow velocity is displayed in brightness. With such a BDF apparatus, abnormalities of the blood flow, such as regurgitation, stricture and shunt blood flow, can be observed in real time. However, the decision whether the blood flow is either in an artery or in a vein or the more detailed inspection such as detection of the absolute value of the blood flow velocity or the extent of the stricture cannot be made. For a detailed inspection, therefore, a spectrum analyzer has been developed which analyzes the frequency spectrum of Doppler shifted reflected waves. In general, as a spectrum analyzing method, a fast Fourier transform (FFT) method which is high in calculation precision is used. The result of the fast Fourier transform is displayed as a blood flow pattern waveform in which the axis of ordinate represents flow velocity and the axis of abscissa represents time. From the pattern waveform the absolute value of and variations with time in the flow velocity can be recognized.
Conventionally the B-mode blood flow imaging apparatus and the frequency analyzer have been independent of each other. Even if they are installed within the same apparatus, they are independent of each other in mode of operation. This is because the direction of transmission of an ultrasonic wave has to be changed at each irradiation in the BDF apparatus or in the BDF mode for obtaining a tomography image, while the direction of ultrasonic-wave transmission is fixed in the FFT apparatus or the FFT mode for obtaining variations in the flow velocity with respect to time at a certain point. Even with a conventional apparatus provided with the two modes of operation, therefore, the position and direction of the blood flow are identified in the BDF mode to determine a desired observation point for the FFT and the operation is then switched to the FFT mode so that the direction of transmission of ultrasonic waves is fixed to the direction of the observation point, a reflection signal from the observation point is range gated to obtain a reflection signal arising from the observation point only and Doppler shifts obtained from the reflection signal are fast Fourier transformed to display a blood flow pattern waveform. In order to observe the blood flow pattern waveforms at several points, therefore, the operation mode has to be switched many times between the BDF mode and the FFT mode, thus providing poor operability and taking a long time to make diagnostic. Furthermore, the detectable maximum value of the Doppler frequencies corresponds to half the repetition frequency (rate frequency) of transmission of ultrasonic waves and thus the part of the waveform above the maximum value will be turned down to the opposite polarity side of the waveform.